Wireless bridges can be employed to bridge two Ethernet Local Area Networks (LANs) over a single radio link, such as an IEEE (Institute of Electrical and Electronics Engineers) 802.11 link. This can obviate the need for wiring and can be useful in situations where wiring is prohibitively expensive. However, wireless bridges tend to be slower so it is often desirable to employ multiple wireless links between two Ethernet LANs.
However, the deployment of multiple wireless bridges introduces other problems. For example, if two (or more) wireless links are coupled to switches to couple a secondary Ethernet LAN to a primary Ethernet LAN, a bridging loop exists unless the spanning tree protocol (STP) used by the switches (e.g. an 802.1D STP) blocks an Ethernet link between the wireless bridges or all but one of the wireless links. However, blocking the Ethernet link between two Ethernet switches can introduce another problem known as “secondary LAN fragmentation”: If the STP root bridge is located in the primary Ethernet LAN and the STP cost of two or more relatively slow wireless links is similar, then the STP protocol will likely break the bridging loop by blocking a high-speed Ethernet link on an Ethernet switch in the secondary Ethernet LAN. Because of fragmentation, packets between two devices on a high-speed secondary LAN may be routed through the primary LAN. For example, because the Ethernet port of a first wireless bridge is blocked, a packet sent by a first device coupled on the secondary Ethernet network to the first wireless bridge to a second device coupled to the secondary network at the second wireless bridge will be routed by the first wireless bridge to the primary Ethernet LAN, and then to the second wireless bridge via the primary Ethernet LAN. Thus, the forwarding path is detoured across two relatively slow radio links. User configurations can be used to resolve the fragmentation problem; however, user configurations in larger more complex networks are not practical.
A Spanning Tree Protocol (STP) organizes a bridged LAN into a loop-free topology; however a STP only permits one link between any two LANS. Currently, link aggregation protocols, such as the IEEE 802.1ad port aggregation protocol, can be used to aggregate multiple links, between two Ethernet switches, into a single, logical spanning tree link, so that the overall bandwidth of the spanning tree link between two Ethernet switches is greatly increased. Current port aggregation protocols have several limitations. The user must explicitly configure the aggregation links; therefore the protocols cannot automatically adapt to lost links and new links. Wireless bridging links are far less stable than wired bridging links. The set of wireless bridging links between any two Ethernet LANs can change. New links can be added to an aggregation bundle and links may be lost from an aggregation bundle.
With existing wired port aggregation protocols, all of the links in an aggregation bundle must be terminated in a pair of Ethernet switches. A first switch, for example, cannot aggregate two links that are terminated in two peer switches. Therefore, an entire aggregated link is susceptible to the failure of a single device. In a wireless network, two wireless bridging links can operate on the same radio channel if the links are separated spatially.
Standard Ethernet switches use source learning, as defined in the IEEE 802.1D MAC Bridges specification, to determine the output port for a target address. The STP establishes a single path for all frames sent between any two stations. If frames from a single Ethernet station are directed over more than one path, then a “path flapping problem” is introduced, where the source-learned path to the station, in a transparent Ethernet LAN, is constantly changing. Current link aggregation protocols avoid the path flapping problem by restricting all links in an aggregation bundle to a single pair of Ethernet switches. However, the path flapping problem cannot be resolved by simply restricting all traffic for a single station to a single aggregation link